1. Field of the Invention
The present invention relates to a multiplexing system for inserting synchronous words to a coded data signal of motion picture image for transmission or recoding of the coded data signal.
2. Description of the Related Art
Referring now to FIG. 15, a general structural example of a conventional coding system is shown. In FIG. 15, a coder 1 takes a difference between an inputted motion picture image data signal from an information source and an estimated value, and efficiently codes the difference.
A run-length coder 2 codes the coded data signal outputted from the coder 1 to a run-length data signal to improve sufficient efficiency in coding.
A synchronous word multiplexer 3 divides the run-length data signal outputted from the run-length coder 2 into a plurality of data signals, inserts a synchronous word per each predetermined interval, and sends the data signal as a coded data signal or sends the data signal to a recorder.
It is general that it becomes necessary to code efficiently in this step, because data volume expands when a motion picture, more particularly, a motion picture image data is transmitted and recorded.
In coding means in order to code efficiently, a motion picture image data signal on one picture and a coded data signal have a hierarchical structure in general.
For example, Both MPEG-1 which is a standard to make a stored media as a main object and MPEG-2 which is a standard of a general coding system having high quality of next generation have a hierarchical structure explained as follows.
A motion picture image data on one picture is composed of three layers of pictures, slices and macroblocks. FIG. 16 shows the three layers of the motion picture image data.
One picture is divided to a plurality of data signals. The picture is divided into a plurality of lines, such as slices 1 to n horizontally, and each slice is divided into a plurality of macroblocks, such as macroblocks MB1 to MBn.
Synchronization of the coded data is established by synchronous words which are attached to each header of a picture and each slice, as shown in FIG. 17. (1) of FIG. 17 shows a structure in which a synchronous word is attached to a header of one picture. (2) of FIG. 17 shows a structure in which a synchronous word is attached to a header of each slice.
In FIG. 17, a length of each macroblock MB is different from those of other macroblocks, so that a length of each slice is also different from those of other slices, because a run-length coding is performed.
Slice size (number of macroblocks in one slice) can be set freely when coding, with due regard to a point of wide use. However, it was general to employ one macroblock line as one slice or fix the number of macroblocks in one slice, for simply coding/decoding in the conventional system. As described above, a synchronous word is attached to each header of a picture and each slice. Moreover, the synchronous word needs many bits in general, so that the number of synchronous words become larger when the slice size become shorter, thus causing deterioration of coding efficiency. Accordingly, it is desired to make the slice size longer as possible, with due regard to coding efficiency. However, it is preferably to make the slice size shorter, even if coding efficiency becomes lower, with due regard to localization of errors when the errors occur in the coded data.